There is a wide variety of RF quadrupole and multipole field devices used for mass spectrometry and related applications. These devices are used for containment, guiding, transport, ion fragmentation, mass (mass-to-charge ratio) selective sorting, and production of mass (mass-to-charge ratio) spectra of beams or populations of ions. Many of these devices are improved versions or variations of the RF quadrupole mass filter and the RF quadrupole ion trap originally described by Paul and Stienwedel in U.S. Pat. No. 2,939,952 (or more accurately in its German counterpart, DE 944 900). The ion trapping and sorting with these devices typically requires the establishment of a relatively intense RF or combined RF and DC electrostatic potential field having predominately a quadrupolar spatial potential distribution or at least one that varies approximately quadratically in one spatial dimension. These fields are established by applying appropriate RF voltages to electrodes shaped and positioned to correspond (at least approximately) to the iso-potential surfaces of the desired electrostatic potential field. Ions constrained in such quadratically varying potential fields have characteristic frequencies of motion which depend only on the intensity and frequency (assuming the RF portion of the field is sinusoidally varying) of the field and the m/z (mass-to-charge ratio—amu/#unit changes) of the ions.
From the earliest stages of the development of the RF quadrupole mass filter and the ion trap, it was realized that the superposition of smaller amplitude AC fields on the RF fields could be advantageous. For example, through careful choice of the frequency composition of these auxiliary fields, specific ion m/zs or m/z ranges could be resonantly excited or destabilized. Typically, these superposed fields are predominately dipolar or quadrupolar in their spatial variation. Early examples of the use of such fields would be the selective detection of ions trapped in a quadrupole ion trap via resonant power absorption, the ejection of specific trapped ion m/zs to an external detector, and selective elimination of abundant ion species from an ion beam transmitted through a mass filter. Auxiliary fields have also been used to selectively modulate a heterogeneous ion beam transmitting through a RF-only operated mass filter in order to create a mass spectrometer [U.S. Pat. No. 5,089,703]. Modern three-dimensional RF quadrupole ion trap mass spectrometers utilize such auxiliary fields to enable mass scanning, mass isolation, and fragmentation of ions [U.S. Re. No. 34,000, U.S. Pat. No. 5,182,451, EP 0336990,5, U.S. Pat. No. 5,324,939].
More recently there have appeared mass selective devices that have the characteristics of both the two-dimensional quadrupole mass filter and the three-dimensional quadrupole ion trap. Such devices are the RF quadrupole ring ion trap and the RF linear quadrupole ion trap. The RF quadrupole ring trap corresponds, in concept, to a two-dimensional quadrupole mass filter bent into a circle such so as to create an extended ion containment region. When used as a mass spectrometer, it is operated in a manner very similar to the conventional three-dimensional quadrupole ion trap. The linear quadrupole trap is a essentially a two-dimensional quadrupole mass filter with a provision to superpose a weak DC potential to provide a trapping field along the axis of the device. These devices may be operated as stand alone mass spectrometers [U.S. Pat. Nos. 4,755,670, 6,177,668]. They also are utilized as ion accumulation devices ahead of RF three-dimensional ion traps, time-of-flight [U.S. Pat. Nos. 5,689,111, 6,020,586] and FT-ICR (Fourier Transform Ion Cyclotron Resonance) mass spectrometers. In more sophisticated hybrid tandem mass spectrometer instruments these devices are used as a first mass analyzer effecting stages of ion accumulation, ion isolation and ion fragmentation before transfer of fragment ions to either a time-of-flight [U.S. Pat. No. 6,011,259] or FT-ICR analyzer for a final stage of mass analysis.
This invention is motivated by and directed to the difficulties presented in applying the auxiliary AC voltages on to the electrodes of a RF linear quadrupole ion trap. However its range of applicability is much broader, as the approach outlined here may be used to superpose auxiliary fields of a variety of spatial geometries on to a main RF field of conventional three-dimensional quadrupole ion traps, RF quadrupole ring ion traps, RF linear quadrupole traps and other inhomogeneous RF field devices where it may be desirable to add auxiliary voltages on to high RF voltage and apply the composite voltages to an electrode.
FIG. 1 shows an example of an electrode structure of a linear quadrupole ion trap, which is known from the prior art. The quadrupole structure includes two pairs of opposing electrodes or rods, the rods having a hyperbolic profile to substantially match the iso-potentials of a two-dimensional quadrupole field. Each of the rods is cut into a main or central section and two end sections. The DC potentials applied to the end sections are elevated relative to that of the central section to form a “potential well” to constrain positive ions axially. An aperture cut into at least one of the central sections of one of the rods is provided to allow trapped ions to be selectively ejected in a direction orthogonal to the central axis in response to AC dipolar electric fields. In this figure, as per convention, the rods pairs are aligned with the x and y axes and are therefore denoted as the X and Y rod pairs. The individual sections of the rod electrodes will be denoted by rod and segment. In the following, the individual rod segments are denoted as X1F-X2F, Y1F-Y2F, X1C-X2C, Y1C-Y2C and X1B-X2B, Y1B-Y2B. For example, the Front, Center and Back sections of the X1 rod are thus denoted as X1F, X1M, and X1B respectively.
FIGS. 2a-2c schematically show the voltages needed to operate the linear ion trap shown in FIG. 1 as a mass spectrometer. These voltages include three separate DC voltages, DC1, DC2 and DC3, to produce the injection and axial trapping fields (FIG. 2a), two phases of primary RF voltage to produce the radial trapping fields (FIG. 2b), and, two phases of AC resonance excitation voltage for isolation, activation and ejection of the ion(s) (FIG. 2c). The necessary combination of the above voltages results in nine separate voltages applied to twelve electrode sections.
A two-dimensional RF quadrupole field is established in the x and y direction by applying a sinusoidal RF voltage, 2VRF Cos(ωt), between the X and Y rod electrode pairs. For most practical devices, the range for angular frequency, ω, of the applied voltage typically corresponds to frequencies of between 0.5 to 2.5 MHz. The amplitude of this main trapping field voltage, VRF, may typically range to exceed 4 KV peak voltage during ion isolation and scanning steps of mass spectrometric experiments. While it is feasible to accomplish this by applying a RF voltage 2VRF Cos(ωt) to only one pair of rod electrodes while maintaining the other pair at RF “ground”, this imposes a RF potential at the axis of the device (bias potential) of VRF Cos(ωt). While this has no effect on ion motion once the ions are within the device, this RF axis potential leads to strong z axis RF potential gradients at the entrance to the device which interfere with the injection of ions from an external source. Symmetric application of voltages VRF Cos(ωt) and −VRF Cos(ωt) to the X and Y rod pairs respectively minimizes the axis potential. However this means that to create the desired superposition of RF, DC and AC fields within the device, corresponding RF, DC and AC voltages must be simultaneously applied to at least some of the electrodes.
In order to enable the superposition of a weak axial DC trapping potential upon the main two-dimensional quadrupole field, each of the four rod electrodes may be divided into segments so as to allow separate DC bias voltages, VDC—FRONT, VDC—CENTER, VDC—BACK, to be applied to the rod segments comprising the Front, Center and Back sections of the structure. These DC rod bias or offset voltages are typically under ±30 volts relative to the instrument “ground” potential. Generally, the voltage difference between center section and end sections needs to be at least a few hundreds of millivolts to effect ion trapping, however voltage differences of 1 to 15 volts are more typically used. In this embodiment of a linear quadrupole ion trap, an auxiliary voltage, 2VAUX(t) must also be applied between the X1 and X2 rods so as to create a substantially dipolar electrostatic field directed along the x axis. Again, as with the main RF trapping voltages, to avoid creating an AC potential on the central axis, its associated z axis voltage gradients at the end of the device, and additionally to avoid creating a substantial AC quadrupole field component, voltages VAUX(t) and −VAUX(t) are applied to the X1 and X2 rods respectively. In this example, the Y1 and Y2 rod electrodes are maintained at AC “ground” (0 volts AC). The functional form of this applied auxiliary AC voltage will depend upon the particular stage of the particular mass spectrometric experiment being performed. In some instances the auxiliary voltage will be sinusoidal and have an angular frequency which will typically be within the range from 0.1×ω/2 to ω/2. At other stages of an experiment, the auxiliary AC voltage may be a broadband waveform that will likely be composed of angular frequencies ranging from 2π×10 kHz to ω/2. The amplitude of this auxiliary AC voltage may range from under 1 volt when it is a sinusoidal (single frequency) wave form, to more than 100 volts when it is a broadband (multi-frequency) wave form. The total voltage applied to the electrode segments will then be the superposition of three voltages. Below are listed the voltages applied to each rod electrode segment.
Electrode SegmentVoltageX1FVX1F = VRFCos(ωt) + VDC—FRONT + VAux(t)X1CVX1C = VRFCos(ωt) + VDC—CENTER + VAUX(t)X1BVX1B = VRFCos(ωt) + VDC—BACK + VAUX(t)X2FVX2F = VRFCOS(ωt) + VDC—FRONT − VAUX(t)X2CVX2C = VRFCos(ωt) + VDC—CENTER − VAUX(t)X2BVX2B = VRFCos(ωt) + VDC—BACK − VAUX(t)Y1FVY1F = −VRFCos(ωt) + VDC—FRONTY1CVY1C = −VRFCos(ωt) + VDC—CENTERY1BVY1B = −VRFCos(ωt) + VDC—BACKY2FVY2F = −VRFCos(ωt) + VDC—FRONTY2CVY2C = −VRFCos(ωt) + VDC—CENTERY2BVY2B = −VRFCos(ωt) + VDC—BACKIn this particular case, the voltages applied to each X rod electrode segment are unique superpositions of the RF, DC and AC voltages. However, as no AC voltage is applied to the Y rod electrodes, delete in this example the voltages applied to the Y rod segment pairs Y1F-Y2F, Y1M-Y2M and Y1R-Y2R are unique only to each pair.
In operation, ions are either formed in or introduced into the volume between the central electrodes. When ions are introduced, the DC voltages on the electrodes of sections X1F-X2F and Y1F-Y2F can be used to gate the ions into the trap volume. After the ions are introduced into the ion trap, different DC voltages are applied to the electrodes of both the front (F) and back (B) sections than that applied to the electrodes of the center section (C) such that ions are trapped in the center section. RF and DC trapping voltages are applied to opposite pairs of electrodes to generate a substantially uniform quadrupolar field such that ions over the entire mass-to-charge range of interest are trapped within the trapping field. Ions are mass selectively ejected from the ion trap by applying a supplemental AC voltage between the X pairs of electrodes of the sections while ramping the main RF amplitude. This supplemental AC voltage generates an electric field which causes ions to be excited or to oscillate with increasing amplitude until they are ejected through the aperture and detected by a detector, not shown.
This current invention is directed to methods and apparatuses for generating voltage superpositions like those shown above and required to operate the linear ion trap. In particular, this invention is directed to an improved circuit for combining an AC voltage with the RF voltage for RF quadrupole and multipole mass filters or ion traps, and more particularly to a circuit which allows the application of AC voltages to the electrodes of RF quadrupole field devices when the AC and RF voltages are simultaneously being applied to the same electrodes.
To explain the problem with existing methods and apparatus one needs to discuss the basic method from the prior art used to simultaneously apply the RF and AC voltages to the rod electrodes. FIG. 3 shows the conceptual schematic of a conventional apparatus for applying the RF and AC voltages to a two-dimensional quadrupole electrode structure. In this example, the rod electrodes are not divided into segments, therefore simplifying our example. However, the basic schemes for applying the RF and AC voltages to the electrodes does not change if the rod electrodes are segmented. FIG. 3 indicates how the X electrode pair AC voltages are combined with the X electrode RF voltage. The RF voltage source 21 drives the primary winding of the tuned circuit RF transformer 22 to produce the X and Y rod high RF voltages at the end connection points of secondary winding 22 of tuned circuit RF transformer 23. The AC voltage source 24 drives the primary winding of AC transformer 25 producing a differential AC voltage across the center tapped secondary winding of AC transformer 25. The high X rod RF voltage connection point of the secondary winding 22 of the RF transformer is connected to the center tap of the secondary winding of AC transformer 26 to add the desired of high X rod RF voltage to the opposing phases of AC voltages produced at the ends of the secondary winding of the AC transformer. The opposing ends of the AC transformer 26 secondary winding are connected correspondingly opposing X rod electrodes and the high Y rod voltage connection point of the RF transformer 23 is connected to both Y rod electrodes. The design requirements for the broadband transformer AC coupling transformer 26 are such that it needs to provide reasonably uniform AC voltage coupling and transformation between its primary and secondary windings over a wide frequency range (about 10 kHz to beyond 500 kHz, assuming ω=2π×1,000 kHz). If broadband multi-frequency AC waveforms are to be used, the amplitude of the voltage across the transformer secondary, 2VAUX, may exceed 150 volts. Although this approach has been successfully used, in many cases a major disadvantage of this approach is that the primary input of the AC transformer 26 is near “ground” potential and the secondary is floated at the RF voltage. Consequently, the primary and secondary windings to the broadband AC transformer must be sufficiently insulated such that the maximum RF voltage applied to the electrodes, VRF—MAXIMUM, can be withstood without voltage breakdown or significant RF power dissipation in the transformer. For a high performance/high voltage system, VRF—MAXIMUM may approach 5,000 volts. All of this RF voltage is dropped between the primary and the secondary windings of the AC transformer
The bandwidth and output voltage requirements for the broadband AC transformer may readily be met using a conventional transmission line type transformer wound on a high permeability toroidal ferrite core and which has modest size (about 2″×2″×1.5″). The additional constraint of having very high RF voltage isolation between the primary and secondary windings greatly complicates the design of such a device and requires a much larger and substantially more expensive AC transformer design.